Electrical contacts are used in a large variety of environments. Several factors may affect the electrical contact. One example of a factor that may greatly affect the electrical contact is a corrosive environment. If the contact material is corroded, for example by oxidation, the contact resistance is usually affected negatively. Corrosion products, like for example electrically insulating oxides or other insulating compounds, lower the surface conductivity of the contact. This in turn results in a lower efficiency of the component of which the electrical contact makes a part.
Another example of a factor that affects the material of an electrical contact is the temperature. The contact may suffer from insufficient mechanical strength or may even weld together due to high temperature. Also, wear may affect the properties of the electrical contact. Furthermore, differences in thermal expansion between different elements in an electrical device may cause thermal stress between the contact material and its adjacent elements, especially if the contact is exposed to thermal cycling.
Naturally, high temperature in combination with a corrosive environment can have an even more detrimental effect on the surface conductivity of the contact material.
Examples of where electrical contact materials may experience high corrosivity and high temperatures are in spark plugs, electrodes, waste, coal or peat fired boilers, in melting furnaces, in vehicles (especially close to the engine), or in industrial environments etc.
Another example of an electrical contact, which is used at high temperatures and in a corrosive environment, is interconnects for fuel cells, especially Solid Oxide Fuel Cells (SOFC). The interconnect material used in fuel cells should work as both separator plate between the fuel side and the oxygen/air side as well as current collector of the fuel cell. For an interconnect material to be a good separator plate the material has to be dense to avoid gas diffusion through the material and to be a good current collector the interconnect material has to be electrically conducting and should not form insulating oxide scales on its surfaces.
Interconnects can be made of for example graphite, ceramics or metals, often stainless steel. For instance, ferritic chromium steels are used as interconnect material in SOFC, which the two following articles are examples of: “Evaluation of Ferrite Stainless Steels as Interconnects in SOFC Stacks” by P. B. Friehling and S. Linderoth in the Proceedings Fifth European Solid Oxide Fuel Cell Forum, Lucerne, Switzerland, edited by J. Huijsmans (2002) p. 855; “Development of Ferritic Fe—Cr Alloy for SOFC separator” by T. Uehara, T. Ohno & A. Toji in the Proceedings Fifth European Solid Oxide Fuel Cell Forum, Lucerne, Switzerland, edited by J. Huijsmans (2002) p. 281.
In a SOFC application the thermal expansion of the interconnect material must not deviate greatly from the thermal expansion of the electro-active ceramic materials used as anode, electrolyte and cathode in the fuel cell stack. Ferritic chromium steels are highly suitable materials for this application, since the thermal expansion coefficients (TEC) of ferritic steels are close to the TECs of the electro-active ceramic materials used in the fuel cell.
An electrical contact material used as interconnect in a fuel cell will be exposed to oxidation during operation. Especially in the case of SOFC, this oxidation may be detrimental for the fuel cell efficiency and lifetime of the fuel cell. For example, the oxide scale formed on the surface of the interconnect material may grow thick and may even flake off or crack due to thermal cycling. Therefore, the oxide scale should have a good adhesion to the interconnect material. Furthermore, the formed oxide scale should also have good electrical conductivity and not grow too thick, since thicker oxide scales will lead to an increased internal resistance. The formed oxide scale should also be chemically resistant to the gases used as fuels in a SOFC, i.e., no volatile metal-containing species such as chromium oxyhydroxides should be formed. Volatile compounds such as chromium oxyhydroxide will contaminate the electro-active ceramic materials in a SOFC stack, which in turn will lead to a decrease in the efficiency of the fuel cell. Furthermore, in the case the interconnect is made out of stainless steel, there is a risk for chromium depletion of the steel during the lifetime of the fuel cell due to diffusion of chromium from the centre of the steel to the formed chromium oxide scale at its surface.
One disadvantage with the use of commercial ferritic chromium steel as interconnect in SOFC is that they usually are alloyed with small amounts of aluminium and/or silicon, which will form Al2O3 and SiO2, respectively, at the working temperature of the SOFC. These oxides are both insulating, which will increase the electrical resistance of the cell, which in turn will lead to a lowering of the fuel cell efficiency.
One solution to the problems that arise when using ferritic steels as interconnect material for SOFC are the use of ferritic steels with very low amounts of Si and Al in order to avoid the formation of insulating oxide layers. These steels are usually also alloyed with manganese and rare earth metals such as La. This has for instance been done in patent application US 2003/0059335, where the steel is alloyed (by weight) with Mn 0.2-1.0%, La 0.01-0.4%, Al less than 0.2% and Si less than 0.2%. Another example is in patent application EP 1 298 228 A2 where the steel is alloyed (by weight) with Mn less 1.0%, Si less 1.0%, Al less 1.0%, along with Y less 0.5%, and/or rare earth metals (REM) less 0.2%.
In U.S. Pat. No. 6,054,231 a superalloy, defined as a austenitic stainless steel, alloys of nickel and chromium, nickel based alloys or cobalt based alloys, is first coated with either Mn, Mg or Zn and then with a thick layer, 25 to 125 μm of an additional metal from the group Cu, Fe, Ni, Ag, Au, Pt, Pd, Ir or Rh. The coating of a thick second layer of an expensive metal such as Ni, Ag or Au is not a cost productive way of protecting already relatively expensive base materials such as superalloys.
US2004/0058205 describes metal alloys, used as electrical contacts, which when oxidised forms a highly conductive surface. These alloys can be applied onto a substrate, such as steel. The conducting surface is accomplished by doping of one metal, such as Ti, with another metal, such as Nb or Ta. Furthermore, the alloys according to US2004/0058205 are applied onto the surface in one step and thereafter oxidised.
None of the cited prior art provides a satisfactory electrical contact material for use in corrosive environments and/or at high temperatures which is produced in a cost-effective manner and with a high possibility of controlling the quality of the conductive surface.
Therefore, it is a primary object to provide a strip material with a low surface resistance and that is corrosion resistant, to be used in an electrical contact.
Another object is to provide a material, which will maintain its properties during operation for long service lives, to be used in electrical contacts.
A further object is to provide material that has a good mechanical strength, even at high temperatures, to be used as electrical contacts in corrosive environments.
Another object is to provide a cost-effective material for electrical contacts.